1. Field of the Invention
The invention relates to optical rotation sensing devices. More particularly, this invention pertains to a method for aligning the path of a light beam transmitted about the optical cavity of a square ring laser gyroscope.
2. Description of the Prior Art
A square ring laser gyroscope is commonly used to measure the angular rotation of a body such as an aircraft. Such gyroscopes have proven to be especially useful as "strapdown" navigation instruments as they simplify system designs by eliminating any need for a gimbaled platform. Operationally, the gyroscope accommodates two counterrotating laser beams which move within a closed square path or ring interior to a generally block-shaped frame under the influence of successive reflections from four corner-mounted mirrors. Upon rotation of the gyroscope about its input axis, the effective path length of each beam changes and a frequency differential is produced between the beams that is nominally proportional to angular rate. This differential is then measured by signal processing electronics to determine the rate.
The conventional gyroscope frame (optical block) includes planar top and bottom horizontal surfaces bordered by eight planar sides that form an octagon-shaped perimeter. Four non-adjacent sides form the mirror mounting surfaces. Each mounting surface must be parallel to the opposed mounting surface and equidistant from the center of the frame to provide an optimum (maximum intensity) beam path. Due to unavoidable manufacturing tolerances of the fabrication process and inaccuracies introduced through polishing of the various surfaces, this ideal condition is not normally realized. As a consequence, a beam path alignment process is required.
Prior art beam alignment processes do not correct for the condition of non-parallelism of opposed mounting surfaces that can result from surface "tilt" with respect to the horizontal surfaces of the gyroscope frame. As a result, the beam path of a square gyroscope (unlike that of a triangular ring laser gyroscope cavity wherein the beam path necessarily defines, and therefore lies in, a plane) is generally non-planar. Typically, the tilt angle of a mounting surface varies between plus or minus six arc-seconds.
The combination of a non-planar beam path with unavoidable stray magnetic fields, possibly generated by nearby electromagnetic devices such as accelerometers, can introduce bias into the gyroscope output. For example, in a forty (40) centimeter ring laser gyroscope without the benefit of this invention an arc-second of tilt angle produces output bias of between 0.01 to 0.04 degrees/hour/gauss. Such bias degrades accuracy and thereby limits gyroscope applications.